In recent years, hybrid electric vehicles (HEV), electric vehicles (EV) and fuel cell vehicles have been manufactured and sold and have been continuously developed from the viewpoint of environmental protection and fuel efficiency improvement. It is essential to make use of chargeable/dischargeable power sources in these so-called electric vehicles. As such power sources, secondary batteries e.g. lithium-ion batteries and nickel-metal-hydride batteries and electric double layer capacitors are used. In particular, the lithium-ion batteries are suitable for use in the electric vehicles because of high energy density and high resistance to repeated charge/discharge cycles. Various developments have been made in the lithium-ion batteries. For use as motor-driving power sources in the vehicles, it is necessary to connect a plurality of secondary batteries in series in order to secure high power output.
When the batteries are connected to each other via connectors, however, the power output of the batteries becomes decreased due to electrical resistance of the connectors. Further, the batteries with the connectors are disadvantageous in terms of space efficiency. That is, the use of the connectors leads to deteriorations in battery output density and energy density.
As a solution to these problems, bipolar secondary batteries such as bipolar lithium-ion secondary batteries have been developed. The bipolar secondary battery has a battery element that includes a plurality of bipolar electrodes, each of which having a collector, a positive electrode active material layer formed on one side of the collector and a negative electrode active material layer formed on the other side of the collector, laminated together via electrolyte layers and separators. It can also be said that the positive electrode active material layer, the electrolyte layer and the negative electrode active material layer constitute one unit cell such that the bipolar secondary battery has a plurality of unit cells are connected in series via the collectors.
In the case of using an electrolyte material containing an electrolytic solution, such as a liquid electrolyte or a polymer gel electrolyte, in the bipolar secondary battery, there arises a problem that the electrolytic solution leaks from the unit cell and causes a liquid junction upon contact with the electrolytic solution of the other unit cell. In order to avoid this problem, Patent Document 1 discloses a bipolar secondary battery in which seal members formed of a polymeric material such as fluorine resin rubber, butyl rubber or silicon rubber are arranged around unit cells so as to seal the unit cells and prevent a liquid junction between the unit cells.